Modern communication systems, and in particular, telecommunication systems, are usually classified according to factors such as the nature of the signal (digital or analog) used by the systems, the type of transmission, the transmission medium, the type of channel, and the nature of the receiver.
The basic objective of any communication system is to ensure that the information entered on the transmission channel is received correctly or, better still, efficiently. The purpose is to obtain, at the receiver, information that is as close as possible to the information transmitted.
If an ideal transmission channel were available, no particular measures would have to be taken to achieve the abovementioned result. However, all transmission channels have to deal with some sort of disturbance. The nature of this disturbance may vary according to the type of channel, the environmental and weather conditions in which the transmission takes place, and the frequency and nature (digital or analog) of the signal. In addition, an important source of disturbance is represented by the possible presence of various transmitters operating in the same environment. The various sources of information thus available are often indistinguishable by the receiver, thus giving rise to problems of mismatching which frequently go well beyond normal problems of interference.
As far as the transmission of digital signals is concerned, such signals expressed in the form of a sequence of symbols may be converted into an analog waveform by a modulation process, and are then transmitted on a corresponding channel. At the receiver, a demodulator reconstructs the transmitted sequence of symbols. Transmission efficiency is normally identified as bit error probability, or bit error rate (BER), i.e., as the number of erroneous bits over the total bits transmitted.
To keep the BER low, it is known that symbol redundancy can be introduced at the upstream of the modulator. In this way, a larger number of symbols than what would strictly be required is transmitted, which leads to an increased band occupation. The redundancy is added by a channel encoder and is removed by a channel decoder at the receiver end. Another approach for reducing the BER includes increasing the amount of energy associated to each symbol. This may be done by transmitting each symbol at a higher power or by protracting the duration of the transmission interval of each symbol.
A communication system may be defined as robust when the information reconstructed at the receiver is correct irrespective of the type of channel, the type of noise present on the channel, and other sources of disturbance. The robustness of a communication system hence represents an element of fundamental importance when the conditions in which the communication is made are unsatisfactory or adverse.
One of the approaches developed for creating robust communication systems involves chaotic systems used in the framework of various transmission schemes. By exploiting the properties of chaotic signals, information is transmitted on a noisy channel by a chaotic encoding with known spread spectrum characteristics and with known advantages in terms of rejection of disturbance, interference and other noisy channel effects.
Basically, two main approaches to chaotic transmission may be distinguished. It is possible to adopt both coherent schemes based on synchronization between the transmitter and the receiver, and non-coherent schemes which are not based on synchronization properties and which prove useful in the case of weak propagation, when synchronization may often get lost. The latter transmission schemes, i.e., the non-coherent ones, are the ones more useful in the case of communications in high-noise environments. They do not call for synchronization between the transmitter and the receiver, and this enables better performance to be provided in the presence of high noise levels.
Over the years, non-coherent schemes have undergone an evolution from very straightforward approaches, such as Chaotic On-Off Keying (COOK) and Chaotic Shift Keying (CSK), towards more complex approaches based on configurations of a differential type, such as Differential Chaos Shift Keying (DCSK) and Frequency Modulation Differential Chaos Shift Keying (FM-DCSK).
In most of the non-coherent techniques the transmitter is required to switch between transmission conditions of different chaotic attractors. This result may be obtained by designing and implementing various chaotic systems, or by using a chaotic system that admits more than one attractor.
In the latter case, switching between the various attractors is normally achieved by fine tuning of the system parameters. Very often this result is neither easy nor straightforward to achieve, especially in the case of chaotic communications where switching between the attractors must be performed in real time. When dealing with real circuits, this corresponds to changing at least one circuit parameter (for example, an RC couple), which results in rather complex operating criteria, particularly in the case of integrated circuits.